Synthetic resins and natural polymeric compounds are used in the production of a great many materials and discrete items whose manufacture includes a curing stage. The manufactured material or item may consist of the polymerized material itself, or the polymerized material may be used as a binder to hold together various layers or aggregates of other materials which in combination with the binder can impart to the finished product its desired properties.
The desired properties are often significantly affected by the degree of polymerization of the resin compound. The affected properties typically include the flexibility and toughness of plastics, or the stiffness, abrasion and impact resistance of laminated sheets.
In various manufacturing processes, the degree of polymerization has been controlled by regulating one or more process parameters such as concentration, residence time, temperature and catalysis. Various instruments have been used to determine heat liberation, viscosity, density and electrical conductivity. Spectroscopic instruments of the gas chromatographic and nuclear magnetic resonance type have been very useful. Laboratory determinations of molecular weights are commonly performed as a quality control measure.
An article by Crandall, E. W. and Jagtap, A. N., entitled "The Near-Infrared Spectra of Polymers", Journal of Applied Polymer Science, Vol. 21, pp. 449-454 (1977) and its bibliography have indicated that near-infrared spectroscopy can be useful in the identification of resins and polymers and in following the course of polymerization and giving some indication of the state of cure. Polymers were melted and pressed between glass plates to give a transparent film, or they were dissolved and their spectra in solution were run against the spectrum of the pure solvent. It was observed that in various polymers the process of curing (with heat) affected the relative intensities of the overtone bands of carbonyl (C.dbd.O), amino and amide N--H and alcoholic O--H. The overtone bands of alkyl and aryl C--H were also observed along with certain C--H combination bands. The intensities of various bands were set forth by way of comparison with that of the C--H stretch band whose fundamental vibration lies at 3.3-3.5 .mu.m in the middle-infrared and whose first overtone appears at 1.7 .mu.m in the near-infrared.
Two of the carbonaceous polymeric materials discussed by Crandall et al are phenol-formaldehyde and urea-formaldehyde resins, which are commonly utilized, inter alia, as binders in glass fiber mats, say, for building insulation. In the manufacture of these mats, typically glass fiber is spun from molten glass and is sprayed with uncured binder compound as the fiber is being showered onto a moving chain conveyor. The conveyor carries the resulting glass fiber blanket through curing ovens wherein the polymeric binder material is exposed to elevated temperatures for an appropriate time period to complete the curing of the binder. After its exit from the ovens, the mat or blanket is cooled by a stream of air from a fan, and certain of its properties may be measured, typically with radiation gauges.
The mass per unit area of the traveling mat has been measured, with various degrees of success, using beta ray gauges, gamma ray or x-ray gauges, or infrared radiation gauges using combinations of wavelengths. The resulting measurements have been used to automatically control the speed of the chain conveyor, thereby determining the amount of coated glass fibers deposited while a section of the conveyor moves through the felting chamber, with the objective of maintaining the weight per unit area of the mat constant along its length.
Various attempts have been made to measure the mass of the binder material per se, for example by taking advantage of the fact that glass is substantially transparent to certain optical (e.g., infrared) and x-ray wavelengths that are significantly attenuated by the binder materials. The objective of this measurement is to be able to control the mass of the binder by regulating the amount or the dilution of the spray material applied.
The degree of cure has been measured in laboratories, for example, by free phenol determination or molecular weight determination. On the basis of their experience with laboratory-analyzed samples, line operators typically make a visual estimate of the degree of cure by inspection of the "color" of the mat, and adjust the oven temperature accordingly. However, as an indicator of cure, color has been shown objectively to be misleading in many cases, as well as subjective. Moreover, color changes do not exhibit high sensitivity except when the mat is already over-cured (burnt).
The degree of cure of the binder is believed to have considerable economic significance, since it affects the property of the glass fiber mat which is termed "recovery". The manufactured mat is usually compressed into rolls or bales for shipment and storage, and "recovery" is the extent to which the mat is able to spring back to its original thickness when the compression is relieved. Recovery is also related to the ability of the mat to maintain its shape and thickness for long periods of time in use as insulation and for other purposes. Hence, if the binder has an optimum degree of cure, a mat with a desired thickness and insulation value in service can be manufactured from a smaller amount of glass fiber and binder. The avoidance of overcuring can also result in lower energy costs during manufacture.
The properties of recovery and resistance to sag and deterioration in service seem to be dependent on an adequate degree of polymerization. There is evidence, on the other hand, that overcuring results in depolymerization as well as other deleterious effects.
It follows that there has been a need for a method and apparatus which provides an instantaneous, substantially continuous and non-destructive indication of the degree of cure of certain traveling polymeric materials.